A basic study of seasonal dating by growth-line counting using Meretrix lusoria (Röding) is discussed in Part I.

1) The growth line of the shell is defined as an increment boundary that appears as a narrow stripe of etchant resisting material. The structural relationship between the crossed lamellae and the growth line of the Meretrix lusoria shell was examined using the scanning electron microscope (SEM). SEM microphotographs of the etched surface with EDTA solution showed that the third order of lamellae were etched into rod-like forms, and that growth lines were rows where the rods were especially dense.

2) A vital staining experiment with alizarin solution was carried out on a tidal flat in the Ariake Sea. Growth lines which are detectable inside the closing surface are thought to be formed with daily periodicity.

3) The annual pattern of daily growth increments of the clams collected monthly from June 1971 to May 1972 showed a gradual decrease in winter toward the growth minimum, suggesting that the standard zero date for a year should be set at the annual growth minimum. This annual growth minimum corresponded to the minimum sea-water temperature, as multivariate analysis indicated that sea-water temperature was the main environmental factor. Summer growth from May to November showed periodic sharp peaks of 14-days' frequency, as estimated by time series analysis.

4) An investigation of growth variation using the recent clams of 6 local groups showed a distinctive decrease in winter, even in the southern specimens. In terms of the life history of this species, counting growth lines is difficult during the early young shell stage (about 0-5 mm in shell height) due to bad preservation of the outer layer and also during the late adult shell stage (shells in their 6th year or later), when shell growth becomes very slow and growth stunts suggesting the spawning frequently occur during summer growth.

5) The collection date of the clam can be estimated by counting daily growth lines from the annual growth minimum to the shell margin; with the growth line count considered to be the number of days from 14 February, within the counting error of. ±18 days.

In Part II, the results of seasonal dating analysis by growth-line counting of the clam Meretrix hisoria (Röding) were decribed for 4 shell layers from shell-midden deposits, 7 shell deposits in abandoned dwelling pits, and 8 shell deposits in other pits excavated from 12 archaeological sites ranging the Earliest Jomon to the Edo periods.

1) The stratigraphic distribution of collection seasons of the clams in shell deposits can be available to distinguish primary deposits from seasonally-mixed deposits. These latter deposits contain several seasons within one block, and the seasons are scattered homogeneously throughout the shell layer. Shell-midden layers in sloping ground were found to be seasonally-mixed deposits. Most of the shell deposits in dwelling pits and other pits seemed to be primary deposits in which the particular collection seasons are clustered stratigraphically.

2) In all these primary deposits only 5 layers were determined to contain shells from only one 1/8-year collection season. The remaining layers contained shells having been collected during several 1/8-year seasons, and some of them showed successive accumula tion over several seasons. In a few of these layers, the time span of accumulation exceeded a one-year cycle.

3) The accumulation rate of the primary deposits was then calculated from the volume of the shell layers accumulated within the largest 1/8-year layer in each deposit. In general, the shell deposits of the Jomon period showed rapid accumulation-for example, 1,000 dm³ in the 1/8-year shell deposit of the Kode dwelling pit 407. Those of the Kofun and Edo periods showed slower rates.

4) Each seasonal assemblage of the 12 sites usually contained shells having been collected throughout the year, suggesting that these sites were used for full one-year cycles as permanent settlements. The general pattern of the seasonal composition of shell collecting activity examined in shell-midden layers shows about 70% of the total shell dated to spring and early summer, then this percentage decreasing gradually in autumn and winter.